Known amplifiers may suffer instability because the wave emitted by each pump is not fully absorbed in the amplifying fiber and therefore comes to disturb the operation of the other pump.
It is known that such instability can be avoided by placing an isolator on the optical path of each of the pumps. Proposals have also been made, for the same purpose, to use two pumps made of different materials, so that one operates at a wavelength of 1480 nm while the other operates at a wavelength of 980 nm.
Such an amplifier is then expensive to make.